I have the 673 receiver, I love it, it is really nice; such a huge improvement in sound over my onkyo receiver.
I cant speak for the sony.
With that said, what your needs for the receiver.
I am using it for movies and gaming
I know the difference between these two isnt that big and a friend of mine has Yamaha rx-v575 which is rly great but I've read rly good things about the sony product... I am just not sure which one is better... since the price is the same for both (~500$)
What kind of speakers do you have to hook up, and is there any extra features you wanted?
I needed to have an independent zone2 out. I can use remote or smart phone to turn on the outside patio seakers; i can play the tv content, any analog source connected to it, pandora, or any dlna music device on the network.
The Yamaha has a front HDMI port and native iPod support so you just use the normal usb cable, no proprietary connection cable like onkyo and most others.
The sony does have more power, and you would have to get the newest model Yamaha to get 4k passthru. Now for me that is not a factor, as the port and video standard will change multiples times agian before there is enough 4k content for it to become the norm. Thus I figure I will be replacing my receiver when I do the upgrade to 4k